The present invention generally relates to an optical scanning system having a barrel toroidal lens, and particularly to an optical scanning system having a barrel toroidal lens suitable for an optical scanning system employed in a laser beam printer, a digital copier, a laser facsimile machine, or the like.
As is well known, it is required to always form a light beam imaging spot of a fixed diameter on a scanned surface in order to effectively perform an optical scan. When the imaging spot diameter changes due to a variation in imaging position during optical scan, the unevenness of the size occurs in a reproduced image. One of reasons which cause variations in imaging spot diameter is curvature of field occurring in an optical scanning system of an optical scanning apparatus. An optical scanning system having a deflection device such as a rotary polygonal mirror (hereafter simply referred to as a polygon mirror) has a problem of cross scan positional error due to facet pyramidal errors of the polygon mirror. Conventionally, an improved optical scanning system is known which intends to correct the facet pyramidal error. In a known optical scanning system, an anamorphic optical scanning system is interposed between a deflection device and a scanned surface. Thereby, the conjugate relationship in view of geometrical optics is approximately established between a deflection surface provided by the deflection device and the scanned surface.
However, in the above-mentioned proposal, the refracting power measured in the sub-scanning direction is larger than the refracting power measured in the main scanning direction. For this reason, when curvature of field in the main scanning direction is corrected, a strong curvature of field in the sub-scanning direction is liable to occur.
Another improvement has been proposed in U.S. Pat. Application Ser. No. 279,096, now U.S. Pat. No. 4,934,772, which discloses a simple optical scanning apparatus which includes a single lens approximately serving as an f.theta. lens and a long toroidal lens. The long toroidal lens used in the proposed scanning system has the function of correcting errors due to curvature of field in the main and sub scanning directions. It is preferable to position the long toroidal lens separately from a scanned surface as much as possible in terms of the utility efficiency of light and mechanical arrangement of optical elements. However, in the long toroidal lens positioned as mentioned above, it becomes difficult to correct curvature of field occurring in the sub-scanning direction when trying to achieve a wider field angle.
Still another example has been proposed in Japanese Laid-Open Patent Application No. 61-120112, which discloses a modified cylindrical lens having a saddle-shaped lens surface as a convex lens surface. This is effective to correct curvature of field occurring in the sub-scanning direction, which prevents the unevenness of the imaging spot diameter.
However, it is very difficult to form the above-mentioned modified cylindrical lens, as described below. The modified lens having a saddle-shaped lens surface is a long lens. Therefore, the shaping and production by using synthetic resin is the practical method of forming the modified cylindrical lens. However, it is very difficult to form a die having a complementary concave surface. Generally, a die having a concave surface is formed by first forming a die having a corresponding convex surface and then transferring the convex surface to a member by casting. However, this process cannot provide a precise surface. It is noted that the above-mentioned Japanese laid-open patent application discloses a cutting production method as a method of forming the saddle-shaped lens surface. However, the mass productivity of cutting production is poor in production of lenses. This prevents the production of a less expensive optical scanning system and thus an economical optical scanning apparatus.